There has been known a working gas circulation engine that is so-called a closed-cycle engine as a conventional engine. In the working gas circulation engine, working gas contained in exhaust gas is allowed to circulate from an exhaust side to a suction side of a combustion chamber, and again can be supplied to the combustion chamber. The working gas circulation engine of this type includes a combustion chamber to which oxygen serving as an oxidizing agent and hydrogen serving as a fuel that is burned by the oxygen are supplied as a reacted product, and to which a working gas having a specific heat ratio higher than that of air is supplied, and a circulation path into which the working gas is circulated from an exhaust side to a suction side of the combustion chamber and again can be supplied to the combustion chamber. The working gas is caused to expand due to the combustion of the oxygen and hydrogen in the combustion chamber, whereby power is generated. Further, the working gas is basically circulated back to the combustion chamber through the circulation path without being released into the atmosphere.
As the conventional working gas circulation engine, a hydrogen engine disclosed in Patent Document 1 described below has been known, for example. In the hydrogen engine disclosed in the Patent Document 1, oxygen and hydrogen are supplied to the combustion chamber, and argon composed of a monoatomic molecule having a specific heat ratio higher than that of air is supplied into the combustion chamber as a working gas in order to enhance thermal efficiency. In the working gas circulation engine, argon is thermally expanded due to the combustion of hydrogen in the combustion chamber, which pushes down a piston to generate power.    Patent Document 1: Japanese Patent Laid-open No. 11-93681